1. Field of the Invention
The present invention relates to a linear actuator.
2. The Prior Art
In beds for hospital and care the surface for carrying the mattress is divided in a back rest section and leg rest section and also typically an intermediate section. The sections for the back and leg can be individually adjusted about a horizontal axis each by means of a linear actuator, cf . e.g., EP 0 498 111 A2 J. Nesbit Evans & Company Ltd.
In certain situations, e.g., in the case of a heart failure, it is crucial momentary to be able to lower the back rest section from an upright position to a horizontal position without the motor and transmission of the linear actuator. For this purpose it is known to equip linear actuators of the “single actuator type” with a quick release, which releases the spindle of the actuator from the motor and at least a part of the transmission. Examples of linear actuators of this type is known from EP 0 577 541 A1, EP 0 685 662 A2, WO 03/033946 A1 and WO 2006/039931 A1 all Linak A/S. It is noticed that the spindle of the actuator is non self-locking, whereby the load on the tube-shaped positioning element which is connected to the spindle nut, starts the rotation of the spindle. The spindle will due to the load accelerate, whereby the positioning element will move towards the end position with increasing speed and the back rest section will in a collision-like manner brake instantly when the back rest section reaches its horizontal position. This is injurious for the patient, who is already traumatized and further it is a severe overload of the bed construction and the linear actuator.
Since the situation by and large is chaotic around the patient, there is in addition a considerable risk that somebody gets caught between the back rest section and the upper frame in which the back rest section is embedded, when it speeds uncontrollably to a horizontal position. This problem has previously been acknowledged in EP 0 944 788 B1 Linak A/S, relating to a linear actuator with a quick release and braking means for controlling the speed of the spindle, when it is released from the motor and transmission. The specific embodiment in EP 0 944 788 B1 deals with a coil spring functioning as a brake spring which tightens against a stationary contact surface. By a controlled loosening of the contact of the coil spring with the contact surface the velocity of the spindle can be controlled. The construction is as far as it goes fine, however delicacy of the operator is required in order to control the speed evenly. In addition the construction is quite complicated. From EP 1 592 325 B1 Dewert Antriebs- and Systemtechnik GmbH a construction is known, where there on the outer end of the tube-shaped positioning element and a fork-shaped front mounting is mounted a worm gear, which drives a cone-shaped element connected to a stationary corresponding cone-shaped braking element. The cone-shaped braking element is spring-loaded in its engagement with the cone-shaped element, which is driven by the worm gear. With an operating handle the cone-shaped braking element can be pulled out of its engagement with the cone-shaped element on the worm gear, whereby the tube-shaped positioning element will start to rotate, causing the spindle nut to start rotating inwards on the spindle. By pulling the cone-shaped braking element more or less out of its engagement with the cone-shaped element on the worm gear, the lowering speed of the tube-shaped positioning element can be controlled.
It is noticed that actuators without a tube-shaped positioning element are also known, but where the spindle nut is embodied as a positioning element and at which the actuator is fastened in the construction, wherein this is built in. An example of such an actuator is known from WO 96/12123 Dietmar Koch. These types of actuators are typically used in armchairs or recliners.
For beds for domestic use a motor drive was developed at the end of the 1980s, where a linear actuator was built into each end of a mutual housing. The motor drive is mounted on the pivot shafts for the back rest section and leg rest section of the bed. These pivot shafts are provided with an arm extending into the housing where it rests against the spindle nut, which is embodied as a sliding element. These dual linear actuators have been developed further such that they also can be used for care beds. Such a dual linear actuator is for example known from WO 89/10715 Eckhart Dewert and DE 38 42 078 A1 Niko Gesellschaft far Antriebstechnik mbH and also WO 2007/112745 A1 Linak A/S. An example of such an actuator with a quick release is dealt with in DE 296 12 493 U1 Dewert Antriebs- and Systemtechnik GmbH Co.